Thus the invention relates to a method for producing fibre composite structures and composite structures obtained by means of VARTM (vacuum assisted resin transfer moulding), where liquid polymer, also called resin, is filled into a mould cavity, in which fibre material priorly has been inserted, as a vacuum is generated in the mould cavity hereby drawing in the polymer. The polymer can be thermoset plastic or thermoplast.
Vacuum infusion or VARTM is a process used for moulding fibre composite mouldings, where uniformly distributed fibres are layered in one of the mould parts, the fibres being rovings, i.e. bundles of fibre bands, bands of rovings or mats, which are either felt mats made of individual fibres or woven mats made of fibre rovings. The second mould part is often made of a resilient vacuum bag, and is subsequently placed on top of the fibre material. By generating a vacuum, typically 80 to 95% of the total vacuum, in the mould cavity between the inner side of the mould part and the vacuum bag, the liquid polymer can be drawn in and fill the mould cavity with the fibre material contained herein. So-called distribution layers or distribution tubes, also called inlet channels, are used between the vacuum bag and the fibre material in order to obtain as sound and efficient a distribution of polymer as possible. In most cases the polymer applied is polyester or epoxy, and the fibre reinforcement is most often based on glass fibres or carbon fibres.
During the process of filling the mould, a vacuum, said vacuum in this connection being understood as an underpressure or negative pressure, is generated via vacuum outlets in the mould cavity, whereby liquid polymer is drawn into the mould cavity via the inlet channels in order to fill said mould cavity. From the inlet channels the polymer disperses in all directions in the mould cavity due to the negative pressure as a flow front moves towards the vacuum channels. Thus it is important to position the inlet channels and vacuum channels optimally in order to obtain a complete filling of the mould cavity.
Ensuring a complete distribution of the polymer in the entire mould cavity is, however, often difficult, and accordingly this often results in so-called dry spots, i.e. areas with fibre material not being sufficiently impregnated with resin. Thus dry spots are areas where the fibre material is not impregnated, and where there can be air pockets, which are difficult or impossible to remove by controlling the vacuum pressure and possibly an overpressure at the inlet side. In connection with vacuum infusion, employing a rigid mould part and a resilient mould part in the form of a vacuum bag, the dry spots can be repaired after the process of filling the mould by for example puncturing the bag in the respective location and by drawing out air for example by means of a syringe needle. Liquid polymer can optionally be injected in the respective location, and this can for example be done by means of a syringe needle as well. This is a time-consuming and tiresome process. In the case of large mould parts, staff have to stand on the vacuum bag. This is not desirable, especially not when the polymer has not hardened, as it can result in deformations in the inserted fibre material and thus in a local weakening of the structure, which can cause for instance buckling effects.
Patent literature discloses examples of the use of a semi-permeable membrane, which increases the area, in which the vacuum is active, and thus reduces the above problems. In this connection the term semi-permeable membrane means a membrane, which is permeable to gasses but impermeable to liquid polymer. Thus if a semi-permeable membrane is placed across the fibre insertion, air pockets can be removed more easily or prevented entirely.
Typically the composite structures comprise a core material covered with a fibre reinforced material, such as one or more fibre reinforced polymer layers. The core material can be used as a spacer between such layers to form a sandwich structure and is typically made of a rigid, lightweight material in order to reduce the weight of the composite structure. In order to ensure an efficient distribution of the liquid resin during the impregnation process, the core material may be provided with a resin distribution network, for instance by providing channels or grooves in the surface of the core material. EP 0 831 987 and EP 0 1304 211 disclose a method and a composite structure, respectively, in which such a core material is used.
As for instance blades for wind turbines have become bigger and bigger in the course of time, and may now be more than 60 meters long, the impregnation time in connection with manufacturing such blades has increased, as more fibre material has to be impregnated with polymer. Furthermore the infusion process has become more complicated, as the impregnation of large shell members, such as blades, requires control of the flow fronts to avoid dry spots, said control may e.g. include a time-related control of inlet channels and vacuum channels. This increases the time required for drawing in or injecting polymer. As a result the polymer has to stay liquid for a longer time, normally also resulting in an increase in the curing time.
WO 2007/098769 describes one method for optimising the process of producing composite structure.
It is an object of the invention is to obtain a new method for producing composite structures, such as sandwich structures, by for instance VARTM processes, and which overcomes or ameliorates at least one of the disadvantages of the prior art or which provides a useful alternative. It is also an object to provide a new and improved composite structure obtainable via the new method.